January 30, 2009--Dark-colored in their more social, swarming state (left) and green when solitary, desert locusts switch into swarm mode when levels of serotonin—a natural chemical also found in humans—spike in their bodies, a January 2009 study says. (Get the full story.)

The insects usually avoid each other, but dire circumstances, such as drought, can force the locusts to swarm.

The bugs transition to swarm mode when they see or smell other locusts, or when their hind legs are jostled.